Resilient architecture: consider building shape

There was a time long ago when you could determine the function, location and even the prevailing climate at the location, just by looking at a building. Those were the now mostly gone days when most buildings were good examples of the form-follows-function dictum.

One can still see examples of buildings with thick tall walls, with very few small window openings, sited on high ground or on the side of a mountain, clearly designed and built for protection against human and natural foes. Or buildings elevated above ground in regions subjected to regular flooding. Buildings with thick walls and windows protected by screens to guard against hot climates. Or buildings with steep sloping roofs in areas of copious rains or heavy snowfalls, and I could go on with others examples, but I am sure we all know what I am talking about.

It is possible for one to compare images of the building types mentioned above with buildings actually seen in hotel strips at international tourism destinations, in waterfront districts and downtown business districts, in places as diverse as Miami, Cancun, Dubai, Johannesburg, Sao Paulo, Moscow, London or Shanghai and come to one conclusion, these modern buildings are so similar and look so much the same in terms of materials and shapes, that they could be anywhere, they are truly interchangeable. Granted that air conditioning has pretty much taken climate out of the equation, and methods and materials of modern construction are quite universal around the world, but this is no reason for not designing for place, local climate, and vulnerability to natural hazards. It is clear there is a homogeneous character to modern architecture worldwide. See the photographs that follow from four big cities (Chicago, Miami, Singapore and Moscow), in four corners of the world far away from each other and draw your own conclusions about the homogeneity of the architecture.

FIGURE 1: Chicago waterfront skyline
FIGURE 2: Miami waterfront skyline
FIGURE 3: Moscow business district
FIGURE 4: Singapore waterfront

So, how did we get to this point? Upon careful consideration of multiple factors it could be argued that it all started with migration of rural populations to urban centers, many of which were already crowded with empty land in short supply. With cities’ population continued growth demand for residential and commercial built-environment soon exceeded existing availability, leaving higher high-rise construction as the only available option to keep pace in fully built-up urban centers. The structure made of steel, or reinforced concrete, and more recently even heavy timber, the skeleton, of these new buildings became the focal point of the design while the building envelope, the skin, depended on glass, masonry, precast concrete, and a variety of aluminum or synthetic panels to clad the exterior. Architects used the colors of various materials, or discontinuities, recesses, projecting elements and changes in the direction of the envelope to create texture, or the shape of the building itself to make a statement that distinguished a project from others around it. The end results of these movements were jungles of structurally similar buildings with a menagerie of looks, crowding urban centers. This has happened around the world, and continues to happen although in some cases for totally different reasons than those mentioned above.

Let us for a moment consider our theory of how this kind of architecture came about, and at the same time let us consider natural hazard vulnerability and climatic differences between the four cities depicted in the photos above. Singapore is barely 140 kilometers north of the equator, it has a tropical and rather warm climate, and it is vulnerable to tropical cyclones, extreme rain events, and flooding. Miami, has a subtropical climate and is located 2700 kilometers farther north from the equator than Singapore. The city is vulnerable to tropical cyclones, extreme rain events and coastal flooding. Chicago is located 4500 kilometers farther north than Singapore, enjoys a temperate climate, and it is vulnerable to windstorms and lake-effect winter storms, snow and icy conditions, as well as tornadoes. And then we have Moscow more than 6000 kilometers farther north than Singapore, with a subartic climate, subjected to winter storms, snow and ice storms, extreme rain and other cold weather extremes. I believe it would be safe to say we all agree there are vast differences in climate and vulnerability to natural hazards between these four cities, because of their individual locations around the globe. This being so, a valid question is: how come the buildings in these photos look pretty much the same despite being located in widely diverse climates, and being vulnerable to different types of natural hazards? Or could we just chalk-off these similarities in design and construction in the face of such climatic and vulnerability differences to the wise use of heating, ventilation, air conditioning and insulating materials?

Perhaps a rather more relevant question would be to ask how each of the types of buildings shown in these photos would perform, when interacting with different types of natural hazards? Answering this question requires visualizing buildings interacting with the damaging components of specific hazards, as well as what happens when the hazard interacts with a conglomerate of buildings in close proximity to one another. What effect do the height, shape, and proportions of the building – the profile it presents to the hazard – as well as the shape and characteristics of the building envelope have, and how these factors affect its performance under a hazard impact?

Regarding visualization, let me digress a bit before discussing this topic. During the seventeen years I taught graduate courses at Florida International University (FIU) and Florida Atlantic University (FAU) on these subjects of hazards, vulnerability, mitigation and risk management most of my students were already practicing design professionals, engineers and architects. Despite their academic backgrounds and professional experience most of my students, especially those with degrees in architecture, expressed surprise that the topics we were discussing in class, which they found highly relevant and critically important to their practices, had not been introduced or discussed while they were pursuing their undergraduate degrees. Furthermore, class discussions revealed most of these professionals had neither been exposed to undergraduate courses discussing impacts of natural hazards on buildings, nor had they been taught or asked to visualize what happens when a building interacts with a hazard, or consider the role of building shape in influencing the performance of a building under hazard impact. Related to this, surveys of educational curricula throughout the country over the years continue to show that most architectural students graduate with a five-year bachelor degree without taking a class on building codes, and the same applies to civil engineering students, but these at least learn about standards for design such as the American Society of Civil Engineers (ASCE) Standard 7 focusing on external environmental loads acting on buildings, and to some degree how building height and shape, as well as the character of the vicinity to a building influence the impacts of such external loads.

In summary our higher education system, save some notable exceptions, has been forming design professionals without equipping them with some critically important tools for their practices, which translates into adverse consequences and higher risk for the way we design and build in vulnerable communities.

With the above in mind let us get back to the topic of visualization. It is clear that various aspects of the building shape can influence the effects of external loads generated by a hazard’s damaging components as it interacts with the hazard itself. Design professionals must be capable of visualizing and recognizing such effects in order to assess the potential for damage and adopt design criteria to protect the structural integrity and functionality of the building.

Let us for example consider wind, the movement of air, as a damaging component of tropical cyclones, tornadoes, and storms. Wind is a fluid in motion that will flow over and around buildings in its path. Therefore a first step in the process of visualization is to determine how aerodynamic the overall shape of the building is; is it a blunt object or a streamlined one? The image below illustrates the basic effects as wind flow around a blunt or a streamlined shape:

FIGURE 5: Wind flow around blunt and streamlined objects

From the illustration we can see how the wind stream separates at it flows around a blunt shape (a) creating a wake and various effects, such as eddies, vortices and turbulence on the leeward side of the object. But when the flow is around a streamlined shape (b) the separation of flow is minimal on the leeward side of the shape. What the design professional must do is visualize what kind of profile the building presents to the flow of wind and what kind of effects will take place from that interaction with the wind, keeping in mind of course that the direction of wind flow may change rapidly or more slowly over time during a hazard event. So the design professional will need to consider what happens when the direction of windflow changes, and determine which may be the worst case scenario.

Once the interaction between the overall shape of the building and wind flow is well understood, we will need to zoom-in and consider more specific details such as the shape of the building envelope. Does the building envelope consists of flat planes with abrupt changes in direction, vertically or horizontally creating 90 degrees or lower-angle corners between adjacent walls or between walls and roof? Or, is the building envelope mainly formed by curving planes? Are there recesses and/or projecting elements, such as balconies, parapets or overhangs, in the envelope? Is the roof flat, or flat with some projecting elements, or is it a sloping roof such as a gable-end or hip roof? Are the exterior walls formed by continuous flat or curved planes, or do they incorporate discontinuities in isolated areas or throughout the entire wall? Does the building have an open-side? How tall and slender is the building? I believe it is clear what the intent is here, we are gathering as much detail as possible about the characteristics of the building envelope, in order to visualize what the actual flow and resulting effects will look like as the building interacts with the wind of a hazard, for example a hurricane.

FIGURE 6: Example of a continuous curving building envelope. There are no recesses, or projecting elements, nor discontinuities
FIGURE 7: Example of a building envelope with major discontinuties, changes in direction of the planes of exterior walls, recesses, and projecting elements.

Figures 6 and 7 above illustrate two quite contrasting types of building envelopes. I believe we would all agree that the building in Figure 6 looks much more like a streamlined object, while that in Figure 7 is definitely a collection of blunt objects. Can you visualize, or imagine, how the wind will flow around each of these buildings during a hurricane? Which will be the more turbulent or the most streamlined flow?

While we engage is this visualization exercise it is critically important that we focus on the fact that we are studying a fluid, the wind, that is applying external loads by way of wind pressure to the building with which it is interacting. This wind pressure can be positive when it pushes against the building envelope, or negative when it suctions on the envelope. Also, the tendency of the wind flow is to remain attached to the surface of the planes of the building envelope, but to separate when it finds a change in direction such as a corner of a wall or the place where exterior walls meets the roof of the building. This attached flow is what we call the boundary layer and a rather important component of wind flow, because it is the behavior of the boundary layer that we are trying to visualize to understand what happens during the wind-building interaction. In doing this the ultimate objective if to characterize the actual impact on wind on a building in terms of actual forces acting on various areas of the building. The direction, magnitude, and complex interaction of these forces with the building envelope and the building structure determine the results of interaction between building and wind. When we understand such results and the potential for damage, we will be in a position to identify alternatives for hazard mitigation solutions for the protection of the building itself, its interiors and contents, and the human life and function that it shelters.

FIGURE 8: This ilustrates the effects of windflow on a building. Red arrows illustrate the direction of the wind flow. Black arrows illustrate the direction of wind-pressure.
FIGURE 9: This illustrates that when wind strikes a particular corner of a building at certain angles vortices are generated that travel along the edges of the roof generating damaging forces
FIGURE 10: Building depicted here has a recessed portion in its building envelope. Red arows illustrate how wind gets “caught’ when flowing into the recess, which causes a momentary “stagnation’ of flow until the ‘push’ of additional wind allows the flow to continue. This phenomenon generates extreme damaging loads in this area of the building.

Two rather important factors to consider during the visualization process are building heigth and tall building slenderness. Main reasons for this are that wind-velocity increases with elevation above ground due to reduced friction to counteract wind flow. The faster flowing wind generates higher wind-pressure in direct proportion to the square of the increase in velocity. From this it follows that high-rise buildings will sustain higher external wind forces on its higher floors and roof than at ground level or lower elevation floors. Also, the proportions of the building in terms of horizontal (plan view) cross section versus the height of the building determine how slender the building is. More slender buildings have lower capacity to resist overturning, drift, and flexure loads, and are therefore subject to a higher potential for damage as they interact with wind. Figures that follow illustrate this.

FIGURE 11: This illustrates how wind flows over and around a tall building in its path resulting in various loadinf effects, such as overturning, uplift, drift, and torsion (not shown here).
FIGURE 12: This basically illustrates the same effects as Figure 11, but adds the torsion effect which happens when you have a non-centric asymetrical structure. The torsion effect is accentuated in slender high-rise buildings.

So, I am sure we all get the idea on what is involved in visualizing the interaction of wind with a building, and how such visualization helps the design professional characterize and quantify the external forces acting on the building envelope, as well as the forces transferred to the building structure. This in turn helps in determining the building’s capability to sustain such forces, and what the potential for damage is based on specific as-built or as-designed design criteria.

We can apply the same visualization methodology to assessing what happens when the building interacts with water, another major damaging component of hurricanes. In the case of water there are others factors to take into account, such as: a) Is it rushing water, such as storm surge or a flash flood? b) Waves, which result from the transfer of wind energy to the water and ride above the mean level of storm surge; c) Effects of the ground such as erosion, undermining, or waterlogging, which generate additional external loads acting on the building.

The same methodology applies to understanding what happens when a particular building shape interacts with other natural hazards including earthquake, floods, tornadoes and others. The overall shape of the building as well as that of the building envelope are key factors in what results from such interactions.

Design professionals, architects in particular, need to pay more attention and understand that when building shape or the building envelope are used to make an architectural statement, something that differentiates this one building from all others around it, this shape and building envelope are also critical factors that determine how the building will perform during a hazard event, and what the potential for damage is.

The higher education sector must also understand the importance of shape in determining building integrity and capability to sustain external loads generated by natural hazards, and introduce this critical topic early in the curriculum allowing students to incorporate it as they complete design projects or participate in research initiatives. By the same token, professional licensing boards must also require that professionals applying for licenses demonstrate a clear understanding of the concept of shape and its importance in influencing building performance during hazard events.

It will truly take educators, regulators, and building design practicioners working together to achieve a paradigm change in building design approach, for the benefit of vulnerable communities everywhere.

When aiming for resilience always consider building shape!

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